1. Field of the Invention
This invention relates, in general, to electronic color printers and, more specifically, to apparatus and methods of storing page data in memory buffers of electronic color printing devices.
2. Description of the Prior Art
Color printers capable of printing pages with text, graphics, and image information can have very demanding memory requirements. In order to obtain the highest quality text and line graphics printing, the data transferred to the printhead must contain all of the necessary information to make the printed data visually accurate. For text and line graphics data, this can be in the form of direct, high resolution "bit-mapped" data with several bits of data associated with each printed pixel. However, for area fill graphics and images, the need for a wider color gamut than the direct bit-mapping system can provide is desirable. This is because more colors can be produced by using half-toning for the rendition of filled area and images.
The conventional approach is to fill a printing buffer using a general purpose CPU to render the full page in memory, and then to print it. To fully utilize a multi-bit gray level printhead in this fashion, four page buffers would be required, one for each color separation of the colors black, cyan, magenta, and yellow. A gray level printhead which can have a four-bit gray scale per pixel for each of the color separations would require a tremendous amount of memory for each page in the buffer. At 400 dots per inch (dpi), a full 11".times.17" page would require approximately 64 Mbytes. Such a size is prohibitive, and it is desirable to store the information in a much smaller memory area without materially detracting from the quality of the finished product.
There are three requirements that need to be met in the design of a data structure capable of providing the memory space needed to economically store multi-bit, gray level, color page data. Along with maintaining the memory at the smallest size possible consistent with maintaining an acceptable level of image quality, it is desirable that the memory or buffer system be expandable from a binary system to a multi-bit printing system. In addition, a desirable implementation of a page buffer memory system would be one in which the data structure keeps the hardware costs to a minimum.
In order to obtain a more efficient memory structure for the page buffer of a color printer, it is important to recognize that the sharpness or resolution of printed information must be stored more precisely than color information so that the printed page will be perceived by an observer as having full memory storage. In other words, it is possible to sacrifice some of the resolution defining the color of the printed data without that sacrifice being perceived by the observer. On the other hand, sharp contrast areas such as text and line graphics require more precise data storage and cannot tolerate the sacrifices acceptable in storing color information. The invention disclosed herein uses these principles.
U.S. Pat. No. 4,270,141, issued on May 26, 1981, recognizes the fact that, although a person can detect delicate variations of brightness across boundaries in a picture, he cannot detect variations of color value as easily. That is, even if the degree of fineness of the color graduations is set much less finely than that of the brightness graduations, the quality of the reproduction will not suffer and no practical inconvenience will be caused. Although recognizing that the perception of color changes is less by an observer, the referenced patent applies this principle to the brightness of the picture information and not to the sharpness of the lines or text represented in the data.
U.S. Pat. No. 4,724,431, issued on Feb. 9, 1988, is a good representation of the conventional state of the art practice of using separate bit maps for each color. The display system in this patent includes three bit map memories for storing pixels representative of a graphic display image. Each of the three bit map memories is utilized to display graphics in a particular color. Additional colors may be displayed utilizing the contents of each bit map memory by combining the colors in each bit map memory. Text information is stored in byte form in a data random access memory.
Therefore, it is desirable, and it is an object of this invention, to provide a page buffer system for electronic color printers which makes efficient use of the memory space without sacrificing significant quality in the printed page. It is also desirable that the memory format be compatible with typical data arrangements received from associated apparatus connected to the electronic printer.